The present invention relates to the field of electronic circuits, and more particularly, to DC-DC converters and associated methods.
Typically, DC-DC converters use current flow information to provide value added functions and features. For example, limiting the current during an overload is commonly implemented as a safety feature. Such a current limit feature would use a signal proportional to output current limiting level. A resistor inserted between the output and the load could generate the desired signal. However, the resistance of this sensor is the subject of a trade-off between power dissipation and signal amplitude. Typically, the signal level at current limit is approximately 0.1 volt, to be well above the noise floor. The sensing resistor""s power dissipation is proportional to the load current at the limit level. At high current levels, the power dissipation can be excessive.
Eliminating the sensing resistor improves the DC-DC converter""s efficiency. Instead of an additional resistive element, current flow is measured using the intrinsic elements within the power converter components. For example, U.S. Pat. No. 5,982,160 to Walters et al. and entitled xe2x80x9cDC-to-DC converter with inductor current sensing and related methodsxe2x80x9d teaches that the current flow information in an inductor can be reconstructed as a voltage across a resistor-capacitor network. This method uses the intrinsic resistance of the inductor""s winding as the current sensing element.
Another method to eliminate the current sensing resistor measures the voltage dropped across the nearly constant, on-state resistance of one of the switching MOSFETs in the converter. The method samples the voltage drop during the conduction interval of the MOSFET to reconstruct the current flow information. Both of these methods make use of the fundamental power converter components as current sensing elements and they avoid using a dissipative element in the power path.
The intrinsic current sensing methods in the above examples can only approximate the actual current flow. These methods suffer in accuracy when compared with the current sensing resistor. For example, utilizing the inductor""s winding resistance as the current sensing element suffers both an initial tolerance error and a variation with temperature. An inductor""s winding initial resistance varies with the length and diameter of the winding""s wire, as well as the specific manufacturing procedure. This same wire resistance increases as a function of temperature. Therefore, the reconstructed voltage signal is a function of the inductor windings"" mechanical tolerance and temperature as well as the current flow.
In view of the foregoing background, it is therefore an object of the invention to provide low power dissipation while accurately measuring and processing current information with thermal compensation in a switching DC-to-DC converter.
This and other objects, features and advantages in accordance with the present invention are provided by a DC-to-DC converter including one or more power switches, a pulse width modulation circuit for generating control pulses for the power switches, and an output inductor connected between the power switches and an output terminal. A thermally compensated current sensor is connected to the output inductor for sensing current in the output inductor. The thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of the output inductor. Also, a current feedback loop circuit cooperates with the pulse width modulation circuit to control the power switches responsive to the thermally compensated current sensor.
The power switches preferably include a low side field effect transistor and a high side field effect transistor connected together. The thermally compensated current sensor may be connected in parallel with the output inductor and may comprise a resistor and a capacitor connected in series. The resistor of the thermally compensated current sensor may be a positive temperature coefficient resistor.
Alternatively, the thermally compensated current sensor may be connected to the at least one power switch for providing a sensed current proportional to a current being conducted through the output inductor. Here, the thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of an on-state resistance of the power switches. Also, in this embodiment, the thermally compensated current sensor is connected between the power switches and the current feedback loop circuit, and comprises a positive temperature coefficient resistor.
Another aspect of the present invention relates to a multiphase DC-to-DC converter having multiple channels. Each of the channels includes a power device with, e.g. a low side power switch and a high side power switch connected together. A pulse width modulation circuit generates control pulses for the power device, and an output inductor is connected between the power device and the output terminal. A thermally compensated current sensor is connected to the power device in each channel for providing a sensed current proportional to a current being conducted through the output inductor. The thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of an on-state resistance of the low side power switch. Also, a current feedback loop circuit cooperates with the pulse width modulation circuit for controlling the power device responsive to the thermally compensated current sensor.
In an alternative embodiment of the multiphase DC-to-DC converter, instead of the thermally compensated current sensor, a feedback resistive network is connected between an input of the control circuit of each of channels and the output terminal. The feedback resistive network includes a negative temperature coefficient resistor having a temperature coefficient that substantially matches a temperature coefficient of an on-state resistance of the monitored power switch of the power devices.
A method aspect of the present invention is directed to regulating a DC-to-DC converter comprising an output terminal, power switches, a pulse width modulation circuit for generating control pulses for the power switches, an output inductor connected between the power switches and the output terminal, and a current feedback loop circuit cooperating with the pulse width modulation circuit for controlling the power switches. The method includes sensing current passing through the inductor using a thermally compensated current sensor connected to the output inductor. Again, the thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of the output inductor. Furthermore, the current feedback loop circuit operates to control the at least one power switch in response to the thermally compensated current sensor.
Alternatively, the method may include providing a sensed current proportional to a current being conducted through the output inductor using a thermally compensated current sensor connected to at least one power switch. Here, the thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of an on-state resistance of the at least one power switch. The current feedback loop circuit controls the at least one power switch in response to the thermally compensated current sensor.